Many types of heavy crude oils contain high concentrations of sulfur compounds, organo-metallic compounds, and heavy, non-distillable fractions called asphaltenes which are insoluble in light paraffins such as n-pentane. Because most petroleum products used for fuel must have a low sulfur content to comply with environmental restrictions, the presence of sulfur compounds in the non-distillable fractions reduces their value to petroleum refiners and increases their cost to users of such fractions as fuel or as raw material for producing other products. In order to increase the saleability of these non-distillable fractions, refiners must resort to various expedients for removing sulfur compounds.
A conventional approach to removing sulfur compounds in distillable fractions of crude oil, or its derivatives, is catalytic hydrogenation in the presence of molecular hydrogen at moderate pressure and temperature. While this approach is cost effective in removing sulfur from distillable oils, problems arise when the feed includes metallic-containing asphaltenes. Specifically, the presence of metallic-containing asphaltenes results in catalyst deactivation by reason of the coking tendency of the asphaltenes, and the accumulation of metals on the catalyst, especially nickel and vanadium compounds commonly found in the asphaltenes.
Alternative approaches include coking, high-pressure, desulfurization, and fluidized catalytic cracking of non-distillable oils, and production of asphalt for paving and other uses. All of these processes, however, have disadvantages that are intensified by the presence of high concentrations of metals, sulfur and asphaltenes. In the case of coking non-distillable oils, the cost is high and a disposal market for the resulting high sulfur coke must be found. Furthermore, the products produced from the asphaltene portion of the feed to a coker are almost entirely low valued coke and cracked gases. In the case of residual oil desulfurization, the cost of high-pressure equipment, catalyst consumption, and long processing times make this alternative undesirably expensive.
Metals contained in heavy oils contaminate and spoil the performance of catalysts in fluidized catalytic cracking units. Asphaltenes present in such oils are converted to high yields of coke and gas which burden an operator with high coke burning requirements. While asphalt markets represent a viable way to dispose of asphaltenes because, normally, no sulfur limits are imposed, such markets are limited in size and location, making this alternative frequently unavailable to a refiner.
Another alternative available to a refiner or heavy crude user is to dispose of the non-distillable heavy oil fractions as fuel for industrial power generation or as bunker fuel for ships. Disposal of such fractions as fuel is not particularly profitable to a refiner because more valuable distillate oils must be added in order to reduce viscosity sufficiently to allow handling and shipping, and because the presence of high sulfur and metals contaminants lessens the value to users. Refiners frequently use a thermal conversion process, e.g., visbreaking, for reducing the heavy fuel oil yield. This process converts a limited amount of the heavy oil to lower viscosity light oil, but has the disadvantage of using some of the higher valued distillate oils to reduce the viscosity of the heavy oil sufficiently to allow handling and shipping. Moreover, the asphaltene content of the heavy oil restricts severely the degree of visbreaking conversion possible due to the tendency of the asphaltenes to condense into heavier materials, even coke, and cause instability in the resulting fuel oil.
Many proposals thus have been made for dealing with non-distillable fractions of crude oil containing sulfur and metals. And while many are technically viable, they appear to have achieved little or no commercialization due, in large measure, to the high cost of the technology involved. Usually such cost takes the form of increased catalyst contamination by the metals and/or the carbon deposition resulting from the attempted conversion of the asphaltenes fractions.
An example of the processes proposed in order to cope with high metals and asphaltenes is disclosed in U.S. Pat. No. 4,500,416. In one embodiment, an asphaltene-containing hydrocarbon feed is solvent deasphalted in a deasphalting zone to produce a deasphalted oil (DAO) fraction, and an asphaltene fraction which is catalytically hydrotreated in a hydrotreating zone to produce a reduced asphaltene stream that is fractionated to produce light distillate fractions and a first heavy distillate fraction. Both the first heavy distillate fraction and the DAO fraction are thermally cracked into a product stream that is then fractionated into light fractions and a second heavy distillate fraction which is routed to the hydrotreating zone.
In an alternative embodiment, an asphaltene-containing hydrocarbon feed is solvent deasphalted in a deasphalting zone to produce a deasphalted oil (DAO) fraction, and an asphaltene fraction which is catalytically hydrotreated in a hydrotreating zone to produce a reduced asphaltene stream that is fractionated to produce light distillate fractions and a first heavy distillate fraction. The first heavy distillate fraction is routed to the deasphalting zone for deasphalting, and the DAO fraction is thermally cracked into a product stream that is then fractionated into light fractions and a second heavy distillate fraction which is routed to the hydrotreating zone.
In each embodiment in the '416 patent, asphaltenes are routed to a hydrotreating zone wherein heavy metals present in the asphaltenes cause a number of problems. Primarily, the presence of the heavy metals in the hydrotreater cause deactivation of the catalyst which increases the cost of operation. In addition, such heavy metals also result in having to employ higher pressures in the hydrotreater which complicates its design and operation and hence its cost.
It is therefore an object of the present invention to provide a new and improved method of and apparatus for upgrading and desulfurizing heavy hydrocarbon feeds containing sulfur, metals, and asphaltenes, wherein the disadvantages as outlined are reduced or substantially overcome.